Laser light generating device and method of fabricating the same

ABSTRACT

In a laser light generating device, the stability against vibration and time-dependent changes will be improved, and influences of temperature changes exerted on the resonator will be reduced.  
     In a laser light generating device ( 1 ) which includes an excitation light source ( 2 ) for generating a continuous-wave excitation light and a solid-state laser resonator ( 4 ) based on using thermal lens effect caused by heat generation at a position of excitation, the solid-state laser resonator further includes a laser medium ( 4   a ), a saturable absorber ( 4   b ), an intermediate medium ( 4   c ) and reflection means ( 4   d ) as the constituents. Influence of vibration is reduced by bonding a substrate of the laser medium ( 4   a ) and a substrate of the saturable absorber ( 4   b ) so as to integrate them. By adopting a configuration which does not need any method of selecting operating point based on temperature changes and is less susceptible to heat, and by relatively moving the excitation optical system and the resonator in the positional relation of the both to thereby adjust the light path length of the resonator, so as to make it possible to select a stable operating point.

TECHNICAL FIELD

With respect to a laser light generating device which has a solid-statelaser resonator including a laser medium, the present invention relatesto a technique of facilitating fine adjustment in the assembly,stabilizing the characteristics, improving the vibration resistance, andmaking it less susceptible to the environment and time-dependentchanges.

BACKGROUND ART

Passive Q-switched or mode-locked pulse light source based oncontinuous-wave excitation has a configuration that includes a lasermedium (Nd:YVO₄, etc.) and a saturable absorber. In particular for thecase where a short pulse is required or a higher recurrence frequency isdesired, the both are often used in contact with each other in order toshorten the resonator length (see Patent Document 1, for example).

Referring now to a small-sized pulsed laser which is constituted of alaser medium and a saturable absorber, the resonator length can beshortened and stabilized by thinning the device thickness through use ofa semiconductor saturable absorber such as SESAM (semiconductorsaturable absorber mirror) or SBR (saturable Bragg reflector), and thisis preferable for the case where a higher recurrence frequency isdesired or a shorter pulse is preferred. This sort of laser resonator isusually configured using a pair of flat mirrors having no curvature, andformed as a stable resonator making use of thermal lens effect withinthe laser medium.

FIG. 8 is a graph which exemplifies an excitation light power dependenceof lens focal length based on the thermal lens effect, where a relationbetween the both is expressed while plotting input power “P_(in)” (unit:W) on the abscissa, and focal length “f” (unit: mm) on the ordinate.

As illustrated in the figure, decreasing trend of f with increase inP_(in) can be found.

A resonator using the thermal lens effect based on temperaturedependence of the refractive index can be formed by making use oftemperature rise at around the excitation center, which is ascribable toconversion of non-oscillating energy to phonon or re-absorption of lightas by-products of absorption of the excitation light at an irradiatedregion of the excitation light.

FIG. 9 is a graph which exemplifies a profile of temperature rise in theradial direction of the laser medium (Nd:YVO₄) caused by the excitationlight, where a relation between the both is expressed while plotting theradius “r” (unit: mm) assuming the excitation center as a referenceposition on the abscissa, and plotting relative temperature change “ΔT”(unit: K) assuming temperature at r=0 as a reference temperature on theordinate (temperature decreases in the direction from the excitationcenter portion towards the peripheral portion).

It is found that the excitation light of approximately 1 W condensed inthe laser medium results in a temperature rise of approximately 200 K atthe excitation center portion (r=0).

Such temperature rise may elevate temperature of the saturable absorberdisposed in close contact with the laser medium and may undesirably varythe characteristics thereof, and this consequently makes it difficult toincrease the output. In short, influence of heat or a high-temperatureregion generated in the laser medium to the saturable absorber maybecome a problem.

To avoid this problem, there is known a configuration in which an airlayer or an intermediate layer is disposed between the laser medium andsaturable absorber (see Patent Documents 2, 3 and 4, for example), andthis makes it possible to reduce thermal influence (degree of heatconduction) of temperature rise of the laser medium exerted on thesaturable absorber.

FIG. 10 schematically shows an example of this type of configuration“a”, in which the laser medium and saturable absorber are disposed in aseparated manner.

An excitation light emitted from an excitation light source “b” advancesthrough an optical system “c” to reach a substrate “d” and irradiate alaser medium “e”.

A saturable absorber “f” opposed to the laser medium “e” is formed on asubstrate “g”, and a gap “h” is formed between the laser medium “e” andsaturable absorber “f” so that an air can exist therebetween. Theopposing planes of the laser medium “e” and saturable absorber “f” arekept in parallel, and this allows variation in the resonator length(light path length) through adjustment of the length of the gap “h”.

Patent Document 1: Japanese Patent Application Publication No.2001-185794 (p.4-7, FIGS. 1 and 7);

Patent Document 2: Japanese Patent Application Publication No.2000-101175 (p.7-8, FIG. 4);

Patent Document 3: Japanese Patent Application Publication No.2001-358394 (p.3-5, FIGS. 1, 3 to 5); and

Patent Document 4: Japanese Patent Application Publication No. 11-261136(p.6-8, FIGS. 1 and 2).

The above-described configuration, however, suffers from a problem ofdegradation of stability due to vibration and time-dependent changes.

In the configuration having the laser medium and saturable absorberindividually fixed on the independent substrates, a change in theresonator length of as much as approximately one-fourth of theoscillation wavelength due to vibration or expansion of an adhesiveunder temperature change can vary the effective gain due to changes inthe oscillation wavelength and relative position of gain spectrum, andthis considerably varies the characteristics such as output and pulserecurrence frequency. This configuration also tends to be readilyaffected by mechanical vibration and causes the jitter to increase, andis associated with a problem that only a small change in the resonatorlength due to time-dependent changes may result in a large variation inthe operating point such as output and pulse recurrence frequency(degradation in the stability).

The configuration having an intermediate matter such as a spacer or shimdisposed between the laser medium and saturable absorber is successfulin avoiding direct heat transfer from the laser medium towards thesaturable absorber, but the heat transfer via the intermediate mattertowards the saturable absorber raises another problem of temperaturerise of the laser medium or saturable absorber which generally have onlya small heat transfer coefficient. Necessity of using a thin spacer alsoraises a problem (infiltration, etc.) caused by surface tension of theadhesive during adhesion, and raises difficulty in the fabrication.

It is therefore a subject of the present invention to improve, withrespect to a laser light generating device, the stability againstvibration and time dependent changes, and to reduce influences oftemperature changes on the resonator.

DISCLOSURE OF THE INVENTION

To solve the aforementioned subject, the present invention relates to alaser light generating device including an excitation light source of acontinuous-wave and a solid-state laser resonator based on using thermallens effect available at a position of excitation in a laser medium,which is characterized by having the following features:

-   -   the solid-state laser resonator is configured so that the        individual substrates having the individual constituents of the        solid-state laser resonator disposed thereon are integrated by        bonding, and so that a reflection means or a saturable absorber        of the solid-state laser resonator is opposed with the laser        medium while placing an intermediate medium in between; and    -   the constituent of the solid-state laser resonator has an        interface inclined from a plane orthogonal to the optical axis        of the excitation light, and the light path length of the        solid-state resonator in the direction parallel to the optical        axis of the excitation light differs depending on setting of the        position of excitation. That is, the light path length of the        resonator can be specified or can be adjusted depending on the        setting of the position of excitation in the direction        orthogonal to the optical axis of the excitation light.

Therefore according to the present invention, changes in the light pathlength of the resonator is less susceptible to vibration or the like, ascompared with a configuration in which the substrates individuallyhaving each constituent of the solid-state laser resonator disposedthereon are supported in an independent manner. It is also made possibleto select an operating point which is stable in terms of thecharacteristics, by adjusting the light path length of the resonatorthrough varying the position of excitation within the laser medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of an exemplary basic configuration of alaser light generating device according to the present invention;

FIG. 2 is a drawing, in cooperation with FIG. 3, of an exemplaryconfiguration of a laser light generating device, showing an excitationoptical system and a solid-state laser resonator;

FIG. 3 is a drawing schematically showing an exemplary configuration ofa solid-state laser resonator;

FIG. 4 is a drawing for explaining light path length of a resonator;

FIG. 5 is an explanatory drawing showing selection of an operating pointthrough moving a resonator on an optical base;

FIG. 6 is a drawing of another exemplary configuration of a solid-statelaser resonator;

FIG. 7 is a drawing of still another exemplary configuration of asolid-state laser resonator;

FIG. 8 is a graph typically showing excitation-light-power dependence ofa lens focal length based on the thermal lens effect;

FIG. 9 is a graph showing an exemplary relative temperature distributionassuming the excitation center as a reference position; and

FIG. 10 is a drawing of an exemplary configuration of a conventionallaser light generating device.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention relates to a solid-state laser resonator based onusing thermal lens effect caused by irradiation of continuous-waveexcitation light, and a laser light generating device equipped with suchresonator, which is available as a light source for display devices(two-dimensional image display device) using GLV (grating light valve)or the like as a linear optical modulation element.

FIG. 1 is a schematic drawing of an exemplary basic configuration of alaser light generating device according to the present invention,assumed as being applied to a passive Q-switched laser.

A laser light generating device 1 has an excitation light source 2 forgenerating continuous-wave excitation light, and the excitation lightfrom the excitation light source 2 is radiated through an optical system3 (simplified as a single lens in the drawing) to a solid-state laserresonator 4. As the excitation light source 2 herein, a semiconductorlaser (laser diode) which is advantageous in downsizing the device isavailable. The optical system 3 (condensing optical system) disposedbetween the excitation light source 2 and solid-state laser resonator 4is configured as an approximately equal-magnification optical system, oras a reduced-magnification optical system for the purpose of convergingthe light from the excitation light source 2.

The solid-state laser resonator 4 has a laser medium 4 a for absorbingthe light from the excitation light source 2 and a saturable absorber 4b, in which a resonator is formed based on using thermal lens effectcaused by heat generation at a position of excitation in the lasermedium 4 a.

On the light path between the laser medium 4 a and saturable absorber 4b, an intermediate medium 4 c is disposed, which is typified by a gaslayer (air layer, etc.) having a refraction of approximately 1. Theintermediate medium 4 c configured as a solid may cause a problem(problem of heat transfer influence, etc.) similar to that arisen in theembodiment in which the spacer is disposed between the laser medium 4 aand saturable absorber 4 b, and a liquid medium is difficult to handle,so that the intermediate medium 4 c is preferably a gas, and is morepreferably air considering influences of the refractive index andsimplification of the configuration.

As reflection means 4 d for forming the resonator (optical resonator), areflective surface, coating film and the like formed on the laser medium4 a and saturable absorber 4 b are available.

As for pulsed light output from the resonator, the wavelength “λ”thereof is specified within a range from 700≦λ≦1600 (unit nm: nanometer)considering taking a typical range of oscillation wavelength of thesolid-state laser. However the applicable range of the present inventionis by no means limited to only the above-described wavelength range.

With respect to a temperature adjusting means for the excitation lightsource and resonator, possible configurations include such as disposingan electronic temperature adjusting instrument composed of Peltierelement or other types of temperature control elements, and such asexcluding this sort of means in order to save the cost.

Alternatively, in order to obtain the continuous-wave laser output,another possible configuration is such as superposing the intermediatemedium 4 c between the laser medium 4 a and the opposing reflectionmeans 4 d, without disposing the saturable absorber 4 b shown in FIG. 1.It is also possible to generate harmonic wave by replacing the saturableabsorber 4 b with a non-linear optical crystal.

The present invention makes it possible to select and set a stableoperating point through adjustment of the resonator light path length,because the resonator light path length in the direction parallel to theoptical axis of the excitation light is not kept constant as describedlater, and instead the resonator light path length varies in thedirection orthogonal to the optical axis of the excitation light.

Because the configuration having the laser medium 4 a and saturableabsorber 4 b individually fixed on the independent substrates is likelyto be affected by vibration or the like, the present invention adopts aconfiguration in which the substrate having the laser medium 4 adisposed thereon and the substrate having the saturable absorber 4 b (orthe reflection means 4 d) disposed thereon are integrated by bonding, sothat the laser medium and saturable absorber are opposed with each otherwhile placing an intermediate medium in between. Possible modes includethe followings:

-   -   (I) a mode in which the substrate of the saturable absorber is        bonded to the substrate of the laser medium in a portion of the        laser medium; and        -   (II) a mode in which the substrate of the saturable absorber            is bonded to the substrate of the laser medium in a portion            having no laser medium formed therein.

FIGS. 2 and 3 are drawings showing an exemplary configuration of thesolid-state laser resonator 4 according to mode (I).

FIG. 2 shows a sectional structure of the resonator at the center, and aview taken in the direction of the optical axis (a drawing as viewedfrom the saturable absorber side) on the right hand side.

A substrate 5 is a plate-like supporting substrate for supporting thelaser medium 4 a, and on one surface of which (the surface opposite tothe incident plane of the excitation light) the laser medium 4 a isdisposed.

A substrate 6 is a supporting substrate for supporting the saturableabsorber 4 b, and formed as a rectangular plate having a recess 6 a. Thesaturable absorber 4 b is fixed by adhesion as being accepted in therecess 6 a. At the center of the substrate 6, a hole 6 b (round hole inthis example) which communicates with the recess 6 a is formed, wheresuch hole is also omissible in another configuration.

The substrates 5, 6 is configured using a transparent base (quartz,sapphire, etc.), where it is preferable to use a material having alarger heat transfer coefficient as compared with that of the lasermedium and saturable absorber. Because the substrates 5, 6 are used asbeing fixed to a supporting member not shown, the attachment surface (orfixation surface) to the supporting member can serve as a heat transfersurface, and this is effective for heat dissipation of the laser mediumand saturable absorber (because heat disposal is facilitated).

FIG. 3 is a drawing schematically showing an essential portion of thesectional structure of the resonator (thickness or the like is shown inan emphasized manner).

The laser medium 4 a disposed on the substrate 5 is composed of asolid-state laser medium doped (implanted) with rare earth elements(Nd:YVO₄, Nd:YAG (Y₃A₁₅O₁₂), etc. doped with neodymium Nd³⁺), and on theincident surface side of which a mirror 7 is formed using a dielectricmulti-layered film or the like, and the on the side opposite to themirror 7 (on the saturable absorber side) an total-reflection (AR)coating is given.

The saturable absorber 4 b is attached to the substrate 6 in such a waythat the circumferential area thereof is adhered to the bottom of therecess 6 a (see portion a in the drawing).

Attachment of the substrate 5 including the laser medium 4 a to thesubstrate 6 is accomplished by adhesion, where the circumferential areaof the laser medium 4 a and the circumferential portion of the openingedge of the recess 6 a are adhered (see portion β in the drawing).

Thus-integrated solid-state laser resonator (or resonator assembly) 4has a gas layer having a refractive index of approximately 1, andpreferably an air layer disposed between the laser medium 4 a andsaturable absorber 4 b.

In view of downsizing, the saturable absorber 4 b is preferably composedof a semiconductor saturable absorber (SESAM, SBR, etc.). For the casewhere the saturable absorber 4 b is configured as an SBR (saturableBragg reflector) having quantum wells formed on a Bragg reflector, thereflective surface thereof is formed as a DBR (distributed Braggreflector). The SBR can intake the light, which is stored in theresonator with the aid of the excitation light into the potential of thequantum well, and so that it can function as a resonator loss underunsaturated conditions, but abruptly reduces the loss and cantransitionally function as a resonator gain switch once a predeterminedamount of light is uptaken into the quantum well.

Once the excitation light emitted from the excitation light source 2passes through from one surface of the substrate to the mirror 7 andradiates the laser medium 4 a, the thermal lens is formed based on thethermal lens effect explained referring to the FIG. 8, and thereby aresonator including the laser medium and saturable absorber is formed.The resonator can output pulsed light in either mode in which the outputlight is emitted in the opposite direction (leftward direction in FIG.3) to the direction of the incident excitation light (rightwarddirection in FIG. 3) (the output light is extracted after being changedin the light path thereof while being reflected by a dichroic mirror orthe like), and in a mode in which the output light is emitted in thesame direction as the direction of the incident excitation light afterpassing through the saturable absorber.

In the present invention, the light path length of the solid-state laserresonator have a continuous distribution along the direction orthogonalto the optical axis of the excitation light. It is to be noted that the“light path length” herein is defined as a sum of quantity which equalsto a refractive index at the laser oscillation wavelength λ multipliedby the a physical length (geometrical length),where the light pathlength of the resonator will be denoted as “L”, and its variation as“ΔL”, hereinafter.

In an exemplary case shown in FIG. 3, the resonator is formed by themirror 7, position of excitation within the laser medium 4 a (positionof heat generation upon irradiation by the excitation light), theintermediate medium 4 c (air layer) and the saturable absorber 4 b, allof which being arranged along the optical axis of the excitation light,where the light path length of the resonator varies along the direction(indicated by arrow “A”) normal to the optical axis. In other words,relative displacement of the optical axis of the excitation light in thedirection indicated by arrow “A” varies the light path length of theresonator corresponding to the change in the position of excitation.

FIG. 4 is a drawing for schematically explaining the change in the lightpath length L, where a trapezoidal portion represents the resonator(sectional form).

The light path length L indicated in the drawing represents the lightpath length on the optical axis (indicated by a solid line) of theexcitation light, where the light path length L becomes shorter if theoptical axis is shifted in the upward direction of the drawing(indicated by arrow “A”) normal to the optical axis (ΔL<0), and becomeslonger if the optical axis is shifted in the downward direction of thedrawing (ΔL>0). This way of positional adjustment and setting isavailable by changing relative positional relation between theexcitation light and resonator.

The constituents of the solid-state laser resonator can be exemplifiedby the laser medium, saturable absorber, intermediate medium andreflection means, and the continuous variation in the light path lengthof the resonator can be given by inclining one of, or two or more ofthese constituents, or by varying the thickness thereof.

Possible modes are as follows:

-   -   (A) a mode in which the laser medium 4 a and the substrate 5        thereof have planes inclined from a plane orthogonal to the        optical axis of the excitation light;    -   (B) a mode in which the saturable absorber 4 b and the substrate        6 thereof have planes inclined from a plane orthogonal to the        optical axis of the excitation light;    -   (C) a mode in which the laser medium and the reflection means 4        attached to the saturable absorber 4 b have planes inclined from        a plane orthogonal to the optical axis of the excitation light;        and    -   (D) a mode in which the thickness, in the direction parallel to        the optical axis, of any one of the laser medium 4 a, saturable        absorber 4 b and intermediate medium 4 c varies in the direction        orthogonal to the optical axis.

First, with regard to modes (A) and (B), relatively inclined planesbetween the mirrors composing the resonator can be formed by adding aslight inclination to the fixation surface of the substrates or thelike, or to the substrate itself. Because the laser resonator is formedbased on using the thermal lens effect as described in the above, therelative shifting of the resonator with respect to the optical axis ofthe excitation light makes it possible to adjust the light path lengthof the resonator and to select an operating point.

The mode (C), in which the reflection means itself has to be added withthe inclination, needs accuracy in the fabrication.

In the mode (D), the thickness of the constituents in the resonator,such as the laser medium, is varied, to thereby vary the difference inthe light path length based on difference in the refractive index withthat of the intermediate medium. It is also allowable to add variationin the thickness of the intermediate medium, to thereby vary the lightpath length of the resonator. In the latter case, it is not alwaysnecessary to incline the interface of the laser medium or saturableabsorber with the intermediate medium, and or to add variation in thethickness thereof. In the exemplary case shown in FIG. 3 in which a part(circumferential area of the opening of the recess 6 a) of the substrate6 is adhered to the laser medium 4 a, it is allowable to press thesubstrate 6 with an uneven force to the laser medium 4 a for fixationthereon, instead of pressing the substrate 6 to the laser medium 4 awith an even force. This results in difference in the thickness of theintermediate medium 4 c between portions applied with a strongpressurizing force and a weak pressurizing force (the thickness issmaller in the portion applied with a strong pressurizing force), andconsequently results in variation in the light path length L.

Other possible modes include combination of two or more of theabove-described modes. It is to be noted that, of these modes, FIG. 1shows a mode in which the thickness “δ” of the intermediate medium 4 cis continuously varied in the direction indicated by arrow “A”, wherethe variation in the thickness “δ” is expressed in an emphasized manner(actual angle of inclination is as much as several milliradians, asdescribed later).

As described in the above, the resonator, which adopts any of theabove-described modes or any combination of these modes, is configuredso as to have a wedge-shaped sectional form when viewed in the planeincluding the optical axis of the excitation light. The light pathlength of the resonator can be added by a method of preliminarily addinga continuous change, or by a method of preliminarily adding a step-wisechange, where the latter is limitative in terms of adjustable range andprocess accuracy. The former way is therefore more preferable in view ofspecifying the light path length of the resonator depending on settingor selection of the position of excitation.

Possible methods of adjusting the light path length of the resonatorinclude the followings:

-   -   (1) a method of shifting the resonator relative to the        excitation light;    -   (2) a method of shifting the excitation light relative to the        resonator; and

(3) a method based on a combination of (1) and (2).

The above-described method (1) refers to a method in which only theresonator is shifted in the direction orthogonal to the optical axiswhile keeping the positions of the excitation light source 2 and opticalsystem 3 unshifted.

FIG. 5 is a schematic drawing for explaining a configuration in whichthe resonator is shifted.

The substrate 5 of the laser medium and the substrate 6 of the saturableabsorber are arranged on a base of the optical system so as to bemovable thereon. Although other possible configuration may be such asallowing a supporting member having the substrates mounted thereon tomove on the base, the drawing depicts the substrates 5, 6 are moved onthe base 8 for simplicity. It is also to be understood that, althoughthe individual substrates are expressed in a separated manner, they areactually moved together because they are integrated as described in theabove.

Both of reference symbols “n1”, “n2” in the drawing indicate normalvectors, where “n1” indicates a normal vector stands on the interfacebetween the laser medium and intermediate medium, and “n2” indicates anormal vector stands on the interface between the saturable absorber andintermediate medium.

The “base surface” of the optical system is a plane in parallel with thesheet of FIG. 5.

A symbol “φ” is an angle between the interface on which the normalvector n1 stands and the interface on which the normal vector n2 stands,which expresses a relative inclination between the both (a larger φvalue corresponds to a larger variation ΔL in the light path length inrelation to the amount of shifting).

In the adjustment of the light path length L, both substrates 5, 6 aremoved on the base 8 along the direction (indicated by the arrow “A”)normal to the optical axis of the excitation light. The adjustment issimple because only requirement is to keep an approximately parallelrelation of the planes including n1 and n2 with respect to the basesurface. More specifically, vector product of the normal vectors n1 andn2 points the direction orthogonal to, or approximately normal to thebase surface, so that movement of both substrates in this direction willnot vary the light path length L. Of course it is allowable to obliquelymove both substrates to a direction inclined by a certain angle from thedirection of the vector product (variation ΔL in the light path lengthwith respect to the amount of movement will be reduced), this maycomplicate the adjustment mechanism and needs further consideration on amethod of positional fixation after the adjustment.

The above-described method (2) refers to a method in which the opticalaxis of the excitation light is shifted while keeping the position ofthe resonator fixed, where possible methods include such as moving itwith respect to either one of the excitation light source 2 and opticalsystem.3, and such as moving it with respect to the excitation lightsource 2 and optical system 3. For the case where the optical system 3is moved, possible methods include such as moving the entire portionthereof, or such as moving only some constituents such as a convergentlens.

As for the above-described method (3), possible methods include such asallocating the methods (1) and (2) to rough adjustment and fineadjustment, for example, and such as applying the methods (1) and (2) ina time-divisional manner.

In short, the light path length of the resonator can be adjusted bychanging the relative positional relation between the excitation lightsource 2 or optical system 3, and the solid-state laser resonator.

The orbital length with respect to the solid-state laser resonator inthis case is preferably 5 mm (millimeters) or shorter. This value isbased on the oscillation limit of the resonator using the thermal lenseffect.

Of the aforementioned constituents of the solid-state laser resonator,those having a plane inclined from a plane orthogonal to the opticalaxis of the excitation light preferably have an angle of the inclinationof 0.07 milliradians or larger and 2 milliradians or smaller.

Similarly for the case where the thickness of the intermediate medium(air layer, etc.) in the direction parallel to the optical axis of theexcitation light is specified so as to vary along the directionorthogonal to the optical axis, it is preferable that the interfacebetween the intermediate medium and the laser medium or saturableabsorber inclines from a plane orthogonal to the optical axis of theexcitation light by an angle of inclination of 0.07 milliradians orlarger and 2 milliradians or smaller.

An angle of inclination of 0.07 milliradians (lower limit value) is aresult of limitation in view of the element size. Assuming now the chipsize as D-mm square, any resonator length is adjustable if theinclination of the interface of the constituents in the resonator issuccessful in obtaining difference in the light path length of as longas half or more of the output wavelength (λ), so that an angle θ of theinclination will be calculated. More specifically, an in equality of[tan θ≧λ/(2·D)] using tangential function (tan) can be given as[θ≧λ/(2·D)] using a linear approximation of [tan θ≈θ]. Assuming now achip size as 5 mm×5 mm, and a minimum wavelength as 700 nm, λ/2=0.35 μmholds, and the calculation gives [0.35 μm/5 mm=0.07 mrad]. The angle ofinclination of the interface of the constituents in the resonator istherefore preferably 0.07 mrad or larger.

An angle of inclination of 2 milliradians (upper limit value) is aresult of limitation in view of the characteristics (lateral modegeneration of the laser, power reduction). In a type of laser of whichresonator is formed by a thermal lens for example, it is calculated thatan approximately 1−W excitation can produce, within the laser medium(Nd:YVO₄), a thermal lens having a focal length of f=5 mm or around. Aresonator length of approximately 0.5 mm results in a mode diameter ofapproximately 50 μm. Assuming that a 20% displacement of the modediameter, or a 10 μm displacement, occurred between the excitation lightand oscillation mode results in generation of the lateral mode orreduction in power, it is necessary to define the upper limit as 2 mrador smaller.

With respect to the light path length L of the resonator of thesolid-state laser resonator, variation ΔL in the light path lengthdefined depending on selection of the position of excitation ispreferably adjusted to half of the laser oscillation wavelength λ (thatis, [ΔL≧λ/2]). This provides a condition for locating the laseroscillation wavelength always at around the center wavelength of thegain, because the vertical mode of the resonator repetitively appearsfor every resonator length of λ/2 at around the laser oscillationfrequency, and is required for selecting a stable operating point.

While adjustment of the light path length of the resonator described inthe above ignored influences of temperature, temperature distributionchanges as much as 200K actually generates in the radial directionassuming the excitation center as a reference position as shown in FIG.9, and a problem of influence of heat will arise.

It is therefore preferable, in view of reducing the thermal stress, thatvalues of thermal expansion coefficient of the substrates 5, 6 are closeto those of the laser medium and saturable absorber. For example, it ispreferable to design the device so that difference in the thermalexpansion coefficients between the laser medium 4 a and substrate 5, ordifference in the thermal expansion coefficients between the saturableabsorber 4 b and substrate 6 falls within a range of ±5×10⁻⁶/K. Thisallowable range is considered as preferable in a practical use, so thata material for the substrate should be selected so that difference inthe thermal expansion coefficient with respect to the laser medium andsaturable absorber does not exceed this range. This type of laser isrepetitively subjected to temperature change between normal temperatureand 200° C. or around when switched between ON and OFF. If thedifference in the thermal expansion coefficient of two substances bondedwith each other is suppressed within a range of ±5×10⁻⁶/K, temperaturechange of approximately 200° C. is only causative of a maximum of 0.1%elongation or shortening, and this is successful in suppressingstress-induced fracture or deterioration.

As for the substrate 5 and substrate 6, values of the heat transfercoefficient are preferably 150 [W/(m·K)] or larger. This is necessary tomake the device less susceptible to temperature change. Morespecifically, this is because a low heat dissipation will undesirablyelevate temperature of the laser medium and saturable absorber, and willadversely affect the characteristics and lifetime. It is to beunderstood that a value of 150 [W/(m·K)] or larger represents acondition such that heat of approximately 1.5 W conducts through asection of 1 mm×5 mm over 5-mm length and causes temperature differenceof 10° C. or less. The above-described value is therefore estimated as anecessary heat transfer coefficient which is calculated based on apractical element size and heating amount, assuming an allowable limitof further temperature rise of 10° C. due to the substrate.

The exemplary case shown in FIG. 2 and FIG. 3 is designed to allow heatof the laser medium 4 a to be dissipated through the substrate 5 andsubstrate 6, and to reduce thermal influence from the laser medium toother portions by virtue of interposition of the intermediate medium 4c. For the case where the laser medium 4 a and saturable absorber 4 bare held using a member such as substrate, it is effective to attach andfix the laser medium 4 a and saturable absorber 4 b on the differentsurfaces as viewed from the direction of the optical axis.

In this example, a part of the substrate 6 and the laser medium 4 a arefixed using an adhesive, and the saturable absorber 4 b is fixed to therecess 6 a of the substrate 6 using an adhesive, which is intended forsuppressing variation in the resonator length ascribable to thermalexpansion of the adhesive (generally having a large thermal expansioncoefficient). More specifically, the direction of thermal expansion ofan adhesive for bonding the laser medium 4 a and substrate 6 in relationto the temperature change (see the arrow in the portion β in FIG. 3),and the direction of thermal expansion of an adhesive for bonding thesaturable absorber 4 b and the substrate 6 in relation to thetemperature change (see the arrow in the portion a in FIG. 3) areoriented to the same direction. For the case where the individualadhesives have nearly equal amount of thermal expansion, variation inthe resonator length ascribable to the individual amount of expansioncan almost be cancelled. Even for the case where the individualadhesives differ in the amount of thermal expansion, the difference inthe resonator length can be reduced (variation in the resonator lengthis in proportion to difference in the amount of thermal expansion).

The conventional method by which an operating point is selected based ona full use of the temperature change is no more available if variationin the resonator length in relation to the temperature change becomessmall, but a stable operating point can be selected by adding variationin the light path length L in the direction orthogonal to the opticalaxis, rather than keeping the light path length L of the resonator atconstant, and by adjusting the light path length corresponding tovariation in the position of excitation (in other words, the resonatoradopts the wedge-formed structure because the adoption of theconfiguration for reducing influences of the temperature changeinevitably requires a configuration for adjusting the resonator lengththrough a technique other than temperature control).

Next, the above-described mode (II) will be described.

FIG. 6 is a drawing of an exemplary configuration according to mode(II), where a sectional structure of the resonator is shown.

In the solid-state laser resonator 4A shown in this example, the lasermedium 4 a is not adhered to the substrate 6, but a portion of thesubstrate 5 having no laser medium 4 a formed thereon is adhered to thesubstrate 6. Because the laser medium 4 a is not adhered to thesubstrate 6, there is no fear of thermal stress exerted on the lasermedium 4 a in the adhered portion therebetween.

The laser medium 4 a is adhered to the substrate 5 on the side havingthe mirror formed thereon, and is housed in the recess 6 a after thesubstrate 5 and substrate 6 are bonded. The saturable absorber 4 b isfixed to the stepped portion on the bottom of the recess 6 a using anadhesive, similarly to as shown in FIG. 3.

The resonator has a wedge-formed structure by providing inclination ofthe planes or variation in the thickness of the constituents thereof, sothat the operating point can be selected by adjusting the light pathlength L of the resonator through shifting of the excitation opticalsystem or resonator in the direction indicated by arrow “A”, withrespect to a relative positional relation between the optical axis ofthe excitation light and resonator.

Because the direction of thermal expansion of the adhesive for bondingthe laser medium 4 a to the substrate 5 in this example is oriented tothe direction opposite to that of the thermal expansion of the adhesivefor bonding the saturable absorber 4 b to the substrate 6, and apositional relation capable of canceling variation in the resonatorlength due to thermal expansion is not established (variation will occurin the direction of reducing the resonator length). The variation in theresonator length can, however, be nearly canceled when the individualadhesives have nearly equal amount of thermal expansion, because thedirection of thermal expansion of the adhesive for fixing the substrate5 to the substrate 6, and the direction of thermal expansion of theadhesive for fixing the saturable absorber 4 b to the substrate 6 areoriented to the same direction (even for the case where the individualadhesives differ in the amount of thermal expansion, the difference inthe resonator length can be reduced).

FIG. 7 is a drawing of another exemplary configuration according to mode(II), where a sectional structure of the resonator is shown.

Differences from the configuration shown in FIG. 6 relate to that thesaturable absorber 4 b is fixed on a small substrate 9, and the smallsubstrate is fixed by adhesion to the stepped portion formed in therecess 6 a of the substrate 6.

The solid-state laser resonator 4B shown in this example has anadvantage of causing only a small thermal stress because the saturableabsorber 4 b is not directly fixed to the substrate 6. Thermal stressexerted to the saturable absorber 4 b or the like can be prevented fromexcessively increasing, if the small substrate 9 has a small size and iscomposed of a material having a good heat conduction property.

The resonator has a wedge-formed structure by providing inclination ofthe planes or variation in the thickness of the constituents thereof, sothat the operating point can be selected by adjusting the light pathlength L of the resonator through shifting of the excitation opticalsystem or resonator in the direction indicated by arrow “A”, withrespect to a relative positional relation between the optical axis ofthe excitation light and resonator.

Because the direction of thermal expansion of an adhesive for bondingthe substrate 5 and substrate 6, and the direction of thermal expansionof an adhesive for bonding the small substrate 9 and substrate 6 areoriented to the same direction, changes in the resonator lengthascribable to the individual amount of expansion can almost be cancelledif the individual adhesives have nearly equal amount of thermalexpansion (or the difference in the resonator length can be reduced evenwhen the individual adhesives differ in the amount of thermalexpansion).

Lastly, a method of fabricating the laser light generation device willbe described.

Major process steps are as follows:

-   -   (i) step of fabricating the solid-state laser resonator; and    -   (ii) step of carrying out alignment of the excitation optical        system with the solid-state laser resonator, and adjustment of        light path length L of the resonator.

First in step (i), the substrate individually having, as being mountedthereon, the laser medium and saturable absorber, which are constituentsof the solid-state laser resonator (assembly), are bonded with eachother. The laser medium and saturable absorber are thus arranged whileplacing the intermediate medium in between, and as described in theabove, the constituents of the resonator are configured so as to have aplane inclined from a plane orthogonal to the optical axis of theexcitation light, or so as to have a thickness in the direction parallelto the optical axis of the excitation light varied in the directionorthogonal to the optical axis.

In step (ii), the excitation optical system including the excitationlight source 2 and optical system 3, and the solid-state laser resonatorfabricated in step (i) are optically aligned. In this process, a stableoperating point is selected by varying the position of excitation in thedirection orthogonal to the optical axis of the excitation light, and byadjusting the light path length of the resonator corresponding to thevariation with an accuracy equivalent to or smaller than the laseroscillation wavelength (this is successful in obtaining desiredcharacteristics with a desirable stability).

For this purpose, the following methods will be used:

-   -   (1) a light detection means (photo-detector, etc.) is disposed        in order to monitor the laser power or pulse recurrence        frequency;    -   (2) the position of excitation of the resonator is gradually        varied while monitoring detection signals from the light        detection means; and.    -   (3) the adjustment is terminated when the power or pulse        recurrence frequency reaches a target value or a maximum value,        or falls within an allowable range containing the target value        or the maximum value, but the adjustment is returned to step (2)        and continued if not.

There are various possible modes for varying relative positionalrelation between the excitation optical system and solid-state laserresonator as described in the above, one exemplary method is to move thesolid-state laser resonator on the base of the optical system asdescribed referring to FIG. 5 (a simple configuration is that theresonator including the laser medium and saturable absorber is moved onthe base surface in the direction parallel to the base surface).

The above-described configuration yields the following advantages:

-   -   nonconformities ascribable to vibration (increase in pulse        jitter, etc.) can be suppressed by adopting a configuration in        which the resonator as a sub-assembly is fixed on the integrated        substrate;    -   vibration resistance is improved by adopting a configuration in        which the light path length of the resonator is less susceptible        to long-term changes, and this makes it less causative of        shifting or fluctuation of the operating point during long-term        operation; and    -   adoption of the resonator having a wedge-formed structure makes        it possible to readily select an operating point and to        fabricate the device, without needing a high accuracy equivalent        to or smaller than the wavelength during the assembly.        Industrial Applicability

As is clear from the above description, according to the inventions ofclaims 1, 18, the light path length of the resonator is less susceptibleto vibration and time-dependent changes, which ensures operationalstability (less in pulse jitter, for example). In addition, adjustmentof the resonator light path length by varying the position of excitationwithin the laser medium makes it possible to select a operating pointwhich is stable in terms of characteristics, and makes it no morenecessary to use a method of selecting the operating point based ontemperature changes. This consequently makes it possible to reducevariation in the resonator length caused by temperature changes (or toadopt a structure less susceptible to heat).

The inventions according to claims 2, 19 can ensure stable operationwhen applied to passive Q-switched laser.

The invention according to claim 3 is successful in radiating theexcitation light through an optical system to irradiate a targetposition.

The invention according to claim 4 is successful in obtaining a laserlight within an output wavelength range of the solid-state laser.

The invention according to claim 5 is successful in raising thereliability of the oscillating operation for the case where theresonator is formed using the thermal lens effect within the lasermedium.

The invention according to claim 6 is advantageous in downsizing thedevice.

The inventions according to claims 7, 8 makes it possible to configurethe resonator using materials of a relatively good availability, needingno materials produced by special methods or no expensive materials.

The inventions according to claims 9, 12, 13, 22 and 23 are successfulin ensuring a resonator length necessary for the adjustment, and inexcluding nonconformities ascribable to generation of lateral mode ofthe laser and reduction in the output.

The inventions according to claims 10, 11 are successful in facilitatingthe fabrication, and in simplifying the configuration.

The inventions according to claims 14, 24 are successful in adjustingthe light path length of the resonator, and in simplifying theconfiguration for the adjustment.

The invention according to claim 15 is successful in reducing influencesof thermal stress.

The invention according to claim 16 is successful in raising the heatdissipation performance.

The inventions according to claims 17, 25 are successful in suppressingvariation in the resonator length due to thermal expansion.

The invention according to claim 20 is successful in adjusting theresonator light path length with a high accuracy.

The invention according to claim 21 is successful in simplifying themethod of adjusting the resonator light path length, needing nocomplicated adjustment mechanism.

1. A laser light generating device comprising an excitation light sourcefor generating a continuous-wave excitation light and a solid-statelaser resonator based on using thermal lens effect available at aposition of excitation in a laser medium, characterized in that: saidsolid-state laser resonator is configured so that individual substrateshaving individual constituents of said solid-state laser resonatordisposed thereon are integrated by bonding, and so that reflection meansor a saturable absorber of said solid-state laser resonator is opposedto said laser medium while placing an intermediate medium in between;and said constituent of said solid-state laser resonator has aninterface inclined from a plane orthogonal to an optical axis of saidexcitation light, and a light path length of said solid-state resonatorin a direction parallel to an optical axis of said excitation lightdiffers depending on setting of said position of excitation.
 2. Thelaser light generating device as claimed in claim 1, characterized inthat: said solid-state laser resonator has, as constituents thereof, alaser medium capable of absorbing said excitation light, a saturableabsorber, an intermediate medium disposed on a light path between saidlaser medium and said saturable absorber, and reflection means forcomposing said resonator; said substrate having said laser mediumdisposed thereon and said substrate having said saturable absorberdisposed thereon are integrated by bonding so as to have a structure inwhich said laser medium and said saturable absorber are opposed whileplacing said intermediate medium in between; and either one or aplurality of said constituents of said solid-state laser resonator haveinterfaces inclined from a plane orthogonal to an optical axis of saidexcitation light, or a thickness of said laser medium or saturableabsorber or intermediate medium in a direction parallel to an opticalaxis of said excitation light varies in the direction orthogonal to saidoptical axis, so as to define a light path length of said resonatordepending on a position of excitation in a direction orthogonal to saidoptical axis of said excitation light.
 3. The laser light generatingdevice as claimed in claim 1, characterized by comprising anapproximately equal-magnification or reduced-magnification opticalsystem disposed between said excitation light source and saidsolid-state laser resonator.
 4. The laser light generating device asclaimed in claim 1, characterized by having an output wavelength of 700nm or longer, and 1,600 nm or shorter.
 5. The laser light generatingdevice as claimed in claim 1, characterized in that an orbital lengthwith respect to said solid-state laser resonator is 5 mm or shorter. 6.The laser light generating device as claimed in claim 2, characterizedby using a semiconductor saturable absorber.
 7. The laser lightgenerating device as claimed in claim 1, characterized by using arare-earth-element-doped solid laser medium.
 8. The laser lightgenerating device as claimed in claim 7, characterized by using Nd-dopedYVO₄ as said solid laser medium.
 9. The laser light generating device asclaimed in claim 4, characterized in that any constituent having aninterface inclined from a plane orthogonal to an optical axis of saidexcitation light, out of all constituents of said solid-state laserresonator, has an angle of inclination from said plane of 0.07milliradians or larger and 2 milliradians or smaller.
 10. The laserlight generating device as claimed in claim 1, characterized by using agas layer having a refractive index of approximately 1 as saidintermediate medium.
 11. The laser light generating device as claimed inclaim 10, characterized by using an air layer as said intermediatemedium.
 12. The laser light generating device as claimed in claim 4,characterized in that: a thickness of said intermediate medium in adirection parallel to an optical axis of said excitation light variesalong in a direction orthogonal to said optical axis; and an interfacialplane between said intermediate medium and said laser medium or saidsaturable absorber is inclined from a plane orthogonal to an opticalaxis of said excitation light by an angle of 0.07 milliradians or largerand 2 milliradians or smaller.
 13. The laser light generating device asclaimed in claim 1, characterized in that, with respect to saidresonator light path length of said solid-state laser resonator, changein said light path length defined depending on said position ofexcitation is a half or more of a laser oscillation wavelength.
 14. Thelaser light generating device as claimed in claim 2, characterized inthat said substrate having said laser medium disposed thereon and saidsubstrate having said saturable absorber disposed thereon are arrangedon a base of an optical system; and in that said laser medium andsaturable absorber are configured as being movable on a base plane ofsaid optical system in such a way that a plane which contains a normalline of an interface between said laser medium and said intermediatemedium, and a normal line of an interface between said saturableabsorber and said intermediate medium, is oriented approximately inparallel with said base plane of said optical system.
 15. The laserlight generating device as claimed in claim 1, characterized in that adifference of thermal expansion coefficient between said laser mediumand its correspondent substrate, or a difference of thermal expansioncoefficient between said saturable absorber and its correspondentsubstrate falls within a range of ±5×10⁻⁶[/K].
 16. The laser lightgenerating device as claimed in claim 1, characterized in that saidsubstrate of said laser medium or said substrate of said saturableabsorber has a value of heat transfer coefficient of 150 [W/(m·K)] orlarger.
 17. The laser light generating device as claimed in claim 2,characterized in that a direction of thermal expansion of an adhesivefor bonding said substrates of said laser medium and said saturableabsorber and a direction of thermal expansion of an adhesive for bondingsaid substrates of said saturable absorber with its correspondentsubstrate are oriented to a same direction.
 18. A method of fabricatinga laser light generating device which comprises an excitation lightsource for generating a continuous-wave excitation light and asolid-state laser resonator based on using thermal lens effect availableat a position of excitation in a laser medium; characterized in thatsubstrates having individual constituents of said solid-state laserresonator disposed thereon are bonded, and that said laser medium andreflection means or a saturable absorber of said resonator are arrangedwhile placing an intermediate medium in between; and in that any of saidconstituent of said solid-state laser resonator is formed so as to havean interface inclined from a plane orthogonal to an optical axis of saidexcitation light, to thereby vary said position of excitation in adirection orthogonal to an optical axis of said excitation light, andthat a light path length of said resonator is adjusted with an accuracyequivalent to or smaller than a laser oscillation wavelength dependingon said variation.
 19. The method of fabricating a laser lightgenerating device as claimed in claim 18, characterized in that: thesubstrates respectively having said laser medium and said saturableabsorber, which configure said solid-state laser resonator, disposedthereon are bonded, and that said intermediate medium is disposedbetween said laser medium and said saturable absorber; and an interfaceinclined from a plane orthogonal to an optical axis of said excitationlight is formed on said laser medium, or on its correspondent substrate,or on said saturable absorber, or on its correspondent substrate, or onreflection means for configuring said solid-state laser resonator; or athickness of said laser medium or of said saturable absorber or of saidintermediate medium in a direction parallel to an optical axis of saidexcitation light is varied in a direction orthogonal to an optical axisof said excitation light; to thereby vary said position of excitation ina direction orthogonal to an optical axis of said excitation light, andthat said light path length of said resonator is adjusted with anaccuracy equivalent to or smaller than the laser oscillation wavelengthdepending on said variation.
 20. The method of fabricating a laser lightgenerating device as claimed in claim 18, characterized in that: saidposition of excitation is varied in a direction orthogonal to an opticalaxis of said excitation light while monitoring changes in output poweror a pulse recurrence frequency of said laser light generating device,and the adjustment of said light path length of said resonator isterminated when said power or pulse recurrence frequency reaches atarget value or a maximum value, or falls within an allowable rangecontaining said target value or said maximum value.
 21. The method offabricating a laser light generating device as claimed in claim 18,characterized in that said light path length of said resonator isadjusted by varying said position of excitation through varying relativepositional relation between an optical system disposed between saidexcitation light source and said solid-state laser resonator, and saidsolid-state laser resonator.
 22. The method of fabricating a laser lightgenerating device as claimed in claim 18, characterized in that saidresonator light path length of said solid-state laser resonator isvaried by a half or more of said laser oscillation wave length byvarying said position of excitation.
 23. The method of fabricating alaser light generating device as claimed in claim 22, characterized inthat a thickness of said intermediate medium in a direction parallel toan optical axis of said excitation light is varied in a directionorthogonal to an optical axis of said excitation light, to therebyincline an interface between said intermediate medium and said lasermedium or saturable absorber from a plane orthogonal to an optical axisof said excitation light, while defining an angle of inclination as 0.07milliradians or larger and 2 milliradians or smaller.
 24. The method offabricating a laser light generating device as claimed in claim 19,characterized in that: said substrate having said laser medium disposedthereon and said substrate having said saturable absorber disposedthereon are arranged on a base of an optical system; and saidsolid-state laser resonator containing both of said substrates is movedwhile keeping a plane which contains a normal line of said planeinclined from said plane orthogonal to an optical axis of saidexcitation light approximately in parallel with said base plane of saidoptical system.
 25. The method of fabricating a laser light generatingdevice as claimed in claim 19, characterized in that said saturableabsorber is fixed by adhesion to said correspondent substrate, and saidlaser medium is fixed by adhesion to said substrate having saidsaturable absorber fixed thereon, so as to orient a direction of thermalexpansion of an adhesive for bonding said laser medium to saidsubstrates of said saturable absorber and a direction of thermalexpansion of an adhesive for bonding said saturable absorber to saidcorrespondent substrate to a same direction.